HX heat transfer analisys

[mfqd]

Hi,

I'm dealing with a dilema regarding my calculations for a plate heat exchanger.

Please take a look to the attached figure: HX_1. There we can see my calculations, where i have in my secondary circuit a temperature regime: 30ºC / 35ºC with a flowrate of 100m3/h. With this i need to define the primary circuit regime.

Then i assumed a flowrate of 40 m3/h for the primary circuit and started to experiment some temperatur regimes. My question relates with this temperature regime. In my idea, as higher is the entrace temperature, the bigger the heat transfer rate should be.
But, with mi calculations i figured not... For example in figure "HX_1" with a entrace temp. of 70ºC, the exit temp. is 12,5ºC lower. In example 2 "HX_2" with the entrace temp. of 110ºC, the exit temp. IS ALSO 12,5ºC.

So, whats going on???... I'm somehow confused....

http://files.engineering.com/getfile.aspx?folder=b053021d-975d-42ce-a5b9-7237389fe925&file=HX_1.jpg

http://files.engineering.com/getfile.aspx?folder=dcfe1520-8e12-4d3a-8b1b-2440ec04c92e&file=HX_2.jpg

 

[FredRosse]

There are two conditions which must always be met:

The energy balance equation: The heat flow rate equals the energy flow into the primary fluid equals the energy flow into the secondary fluid. Your calculations indicate this, but assume a fixed value of heat flow, hence the same temperature rise in either fluid.

The transport equation: The heat flow is governed by the heat transfer coefficients within the heat exchanger, the fluid mass flowrates, and the temperatures for the fluids. Heat flow equals the overall heat transfer coefficient (a complex function of the fluid properties, the fouling resistances, the plate metal and thickness) times the heat transfer surface area times the LMTD (Log mean temperature difference).

You must simultaneously satisfy both equations, and a unique solution exists for any combination of inlet flows and temperatures for both the primary and secondary fluids.

 

[IRstuff]

Both conditions are as you specified. You determined the maximum heat exchange with the 5°C secondary rise and 100 cmh flow.

You arbitrarily forced the output temperatures to be constant, hence, limiting the heat exchange to a fixed value, which means that your primary loop will have always have the same 12.5°C drop, regardless of the absolute temperature.

The more normal way to solve this problem is to determine what the primary outlet temperature is supposed to be, and to solve for what the secondary loop must do to satisfy that constraint.

TTFN

FAQ731-376